Multimode optical fibers with increased bandwidth

ABSTRACT

The specification describes a technique for drawing circular core multimode optical fiber using twist during draw to increase fiber bandwidth.

RELATED APPLICATION

This application is a division of application Ser. No. 10/027,067, filedDec. 20, 2001, now U.S. Pat. No. 6,735,985, issued Mat 18, 2004.

FIELD OF THE INVENTION

This invention relates to multimode optical fibers having increasedbandwidth.

BACKGROUND OF THE INVENTION

Manufacture of state of the art multimode optical fiber requiresdemanding control over a variety of power loss and signal impairmentmechanisms. For multimode fiber, controlling mode dispersion is animportant goal.

As is well known, within the optical fiber, bits of data are representedby pulses of light. Each pulse of light will spread, or disperse, overtime as it travels the length of the fiber. If these data pulsesoverlap, they can no longer be unambiguously read at the receiving end.Lower tendency toward data pulse overlap results in higher datatransmission capacity, i.e. higher bandwidth. Therefore, the bandwidthof optical fibers is ultimately limited by dispersion.

Predominant forms of dispersion are chromatic dispersion and modedispersion. Chromatic dispersion is well known and occurs is all opticalfiber systems. Mode dispersion, or intermodal dispersion, occurs mainlyin multimode optical fibers where the large core diameter allows a widenumber of optical paths for light to travel. Different optical pathsusually have different lengths. Because the modes travel along paths ofvarying length, they arrive at the fiber end at different intervals oftime. If the time difference is great enough, the pulse traveling thefaster path will overlap the pulse ahead of it.

Multimode optical fiber bandwidth is optimized by minimizing intermodaldispersion. This is commonly achieved by using graded index profileswherein the refractive index gradually increases from the outer regionof the cladding to the center of the core. Signals travel faster in thelow-index region near the cladding, and slower in the high-index regionnear the center of the core.

In multimode optical fiber, mode dispersion may be referred to asDifferential Mode Delay (DMD). Manufacturing specifications for opticalfiber for use in state of the art systems have rigid requirements forDMD. The DMD specification within a given optical fiber core radius iscalled mask width. For example, fibers with DMD of less than 0.23 ps/mwithin a core radius of 18 microns are referred to as having a maskwidth within the 18 micron radius of <0.23 ps/m. This may also beexpressed as MW 18<0.23 ps/m. These mask width specifications correspondto an optical fiber having an Effective Modal Bandwidth (EMB) of 2000MHz-km at 850-nm, and optical fibers meeting these mask widthspecifications typically have overfilled bandwidths >500 MHz-km at1300-nm. 850-nm and 1300-nm are typical wavelengths of choice formultimode optical systems. Manufacturing multimode optical fiber to meetthese specifications has proven difficult.

It is known that short range refractive index variations, orperturbations, cause mode mixing, which has the effect of averaging thetransmission distance for all modes traversing the optical fiber.Techniques for enhancing mode mixing have been sought by workers in theart to address adverse DMD. One of these is described in U.S. patentapplication Ser. No. 847,034 filed May 1, 2001 by DiGiovanni et al.,which is incorporated by reference herein in its entirety. In thatapproach the fiber core is made non-circular, and the optical fiber istwisted during draw. Significant increases in bandwidth result. However,there is always a quest for further improvements in multimode opticalfiber bandwidth.

STATEMENT OF THE INVENTION

We have developed multimode optical fiber that satisfies currentbandwidth needs and has promise for meeting future high-speed Ethernetprotocols. It is known from prior work, e.g. the patent referencedabove, that short range refractive index variations, or perturbations,in a multimode optical fiber can result in enhanced mode mixing. Theseperturbations exist as inherent “defects” even in high quality opticalfiber. We have recognized the ubiquitous nature of these perturbations,as well as the fact that to be optimally effective they should berandomized along the path lengths of the various modes traversing thefiber. Following this recognition we have demonstrated that DMD inmultimode optical fibers with cores regarded as essentially circular canbe reduced by adding twist to the optical fiber as it is drawn.

The invention will be described in greater detail with the aid of thedrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a multimode optical fiber thatmay be manufactured in accordance with the invention;

FIG. 2 is a diagram of idealized mode transmission along a length ofoptical fiber;

FIG. 3 is a diagram showing three types of potential spatialnon-uniformities in the glass material of the optical fiber;

FIG. 4 is a schematic view of an optical fiber drawing apparatussuitable for use with the invention;

FIG. 5 shows, schematically and in top view, the guide portion of theapparatus of FIG. 4;

FIGS. 6–8 are representations, also schematically and in top view, ofguide means that can be used to practice the invention; and

FIGS. 9 and 10 show measured data comparing bandwidth for optical fiberstreated with spin according to the invention with untreated opticalfibers.

DETAILED DESCRIPTION

Referring to FIG. 1, a typical multimode optical fiber structure isshown with core 11 and cladding 12. A standard polymer protectivecoating, typically an acrylate polymer, is shown at 13.

The multimode characteristic of the optical fiber is represented by therelative size of core 11, and the shaded representation of the indexvariation from the center to the outside of the core. There are threecommon types of multimode fiber. These may be defined by thecore/cladding diameter, expressed in microns:

-   -   50/125—FDDI standard; used for data communications;    -   62.5/125—FDDI standard for local data communications (and the        most prevalent multi-mode fiber in use in North America);    -   100/140—designed for specialized applications where light        coupling efficiency and bending sensitivity are important.

As a general proposition, multimode fiber has a core/clad diameter ratiogreater than 0.2, and typically greater than 0.4. This can be comparedto single mode fiber with a core/clad diameter ratio typically less than0.1. The core itself in a multimode fiber is typically greater than 30microns, while the core of a single mode fiber is typically less than 10microns.

The mechanism of mode mixing for reducing DMD is illustrated in FIG. 2.It should be understood that this is a highly idealized representationand is presented only for a phenomenological understanding of thisaspect of the invention. The core of the optical fiber in thisillustration is represented by 21. Two modes are represented, mode A,shown as a solid line, and mode B, shown as a dashed line. Thepropagation direction is toward the right of the page. Mode A starts topropagate near the outside of the core as shown to the left of thefigure, and mode B starts propagation near the core center. It is easilyunderstood that if these relative mode propagation patterns continuedown the length of the fiber, mode A would travel a distance substantialgreater than mode B. At the center of the diagram is a region, denotedby reference numeral X, where mode mixing occurs. In reality, modemixing occurs over the entire length of fiber and is due to coupling ofenergy from one mode to another. That coupling is enhanced by theperturbations mentioned above. With the schematic representation of modemixing at X in the figure, mode A thereafter is coupled to a mode nearerthe center of the core, and mode B couples to a mode near the outside ofthe core. While this diagram is highly idealized, it does serve toillustrate how, if modes constantly couple or mix along the length ofthe fiber, the average distance traveled by all modes over an extendedlength L will be equal. In a real environment, mode mixing occurs bothwithin a mode group, as well as between mode groups. For a discussion ofmode coupling in multimode optical fibers see R. Olshansky, Reviews ofModern Physics, Vol. 51 (2), pp. 341–367 (1979), which paper isincorporated herein by reference.

Optical fiber preforms useful for drawing optical fiber according tothis invention may be made by a variety of known techniques includingMCVD, PCVD, OVD and VAD. The preferred technique is MCVD. In MCVD, aprecisely controlled mixture of chemicals flows through the inside of arotating pure silica glass tube. The silica tube is heated externally bya torch. The gases flowing through the tube react to form high puritysilica particles in the vicinity of the deposition torch. As theparticles are formed, they deposit downstream on the inner wall of thetube. The particles are heated to a high temperature to consolidate theminto pure transparent silica. The tube is then collapsed and is readyfor optical fiber draw.

Broadly speaking, the invention is embodied in a novel and convenientmethod of making multimode optical fiber having improved DMD, and theresulting product, i.e. the improved optical fiber itself. Morespecifically, the inventive method comprises providing a conventionaloptical fiber preform, heating at least a portion of the preform to aconventional draw temperature, and drawing optical fiber from the heatedpreform in such a way that a spin is impressed on the fiber. The spin inthe fiber is produced by applying a torque to the fiber while it isbeing drawn, i.e. in the softened glass state, such that the fiber iscaused to twist around its longitudinal axis, with a resulting torsionaldeformation of the glass. When the softened glass cools, the deformationis frozen in the fiber, such that the fiber exhibits a permanent “spin”,i.e., a permanent torsional deformation. The existence of such afrozen-in spin can be readily ascertained, e.g., by means of a travelingmagneto-optic modulator, as used by M. J. Marrone et at., OpticsLetters, Vol. 12(1), p. 60.

Associated with the frozen-in spin is a pitch, the spin repeat distancealong the fiber. The pitch can be expressed in terms of twists permeter, and, for the purpose of this invention, is at least 4 twists permeter.

In the preferred embodiment of the invention, the twist is alternatedbetween a left hand twist and a right hand twist, i.e. the fiberrelative to the longitudinal axis is rotated clockwise, thencounterclockwise. Rotations are complete 360 degree rotations so aprescription for, e.g., 4 twists per meter, would involve 2.5 rotationsin each direction. This produces a fiber with a chiral structure, chiralbeing defined as a right hand twist followed by a left hand twist. Theperiod of twist reversal can be expressed as twists per meter, asmentioned above, or in terms of the length of fiber between twists. Forexample, a fiber with 5 twists per meter would have a twist (or chiral)period of 20 cm. In practice, the twist can be imparted by rotating thepreform during draw, or by rotating the drawn (solidified) fiber duringdraw. Typically, the number of twists in the clockwise direction will bethe same as the number in the counterclockwise direction. However,different numbers in each direction may also be used to advantage.Although it is preferred to alternate twists between clockwise andcounterclockwise directions, similar advantages may be realized usingcontinuous twist, i.e. twist either in the clockwise or thecounterclockwise direction.

The effect of twisting the fiber back and forth is to further randomizethe perturbations experienced by a given mode traveling along the coreso that, ideally, every mode in every mode group experiences a maximumamount of mode coupling. It may be preferred to use a random twistperiod, i.e. to change the twist frequency as the fiber is being drawn.This is consistent with known theory that predicts more effective modemixing with random perturbations. See, for instance, S. C. Rashleigh, J.of Lightwave Technology, Vol. LT-1(2), pp. 312–331, especially p. 320,where it is stated that,

-   -   “ . . . regardless of the actual distribution f(z) of the        birefringence perturbations, only the one spectral component        with frequency b_(i) can couple the two polarization eigenmodes.        All other spectral components do not efficiently couple the        modes”.

The parameter b_(i) is the intrinsic birefringence of the fiber, andF(b_(i)) is the Fourier transform of f(z). Since the perturbation f(z)is essentially random, it is theorized that a constant pitch spin willnot result in efficient mode coupling. On the other hand, non-constantpitch spin, especially spin that has alternately positive and negativehelicity, is likely to contain spatial components that produce efficientcoupling. Strong coupling can be obtained with spin of varying spatialfrequency that comprises, in addition to regions of relatively high spinspatial frequency, regions of relatively low spin spatial frequency.This is, for instance, the case if the spin alternates between positiveand negative helicity.

Spatial perturbations that can be randomized to advantage according tothe invention are illustrated by the diagram of FIG. 3 where the circle23 represents the cross section of the optical fiber. Longitudinalperturbations are represented by arrow 24, azimuthal perturbations arerepresented by arrow 25, and radial perturbations are represented byarrow 26. U.S. patent application Ser. No. 847,034 uses elliptical corestructures to increase mode mixing. The twist imparted to the fibermakes the optical path length equal for any random walk along the fibercore. However, we have recognized that in fibers with circular cores,twist can improve DMD by randomizing other perturbations, for example,azimuthal variations. For the purpose of this invention, circular coresare intended to cover core geometries with ovality less than 6%.

The invention is also embodied in silica based optical fiber per se,comprising a circular core and a cladding, with the core having largereffective refractive index than the cladding material that surrounds thecore, and further wherein the core contains a chiral structure accordingto the invention. The chiral structure is produced by modifying theoptical fiber draw apparatus in a manner described below.

FIG. 4 shows an optical fiber drawing apparatus with preform 31, andsusceptor 32 representing the furnace (not shown) used to soften theglass preform and initiate fiber draw. The drawn fiber is shown at 33.The nascent fiber surface is then passed through a coating cup,indicated generally at 34, which has chamber 35 containing a coatingprepolymer 36. The liquid coated fiber from the coating chamber exitsthrough die 41. The combination of die 41 and the fluid dynamics of theprepolymer, controls the coating thickness. It is important that thefiber be centered within the coating cup, and particularly within theexit die 41, to maintain concentricity of the fiber and coating.Hydrodynamic pressure in the exit die itself aids in centering the fiberin the die. The prepolymer coated fiber 44 is then exposed to UV lamps45 to cure the prepolymer and complete the coating process. Other curingradiation may be used where appropriate. The fiber, with the coatingcured, then passes an assembly of reels for aligning, pulling, andtaking up the fiber for storage on spool 56. The drive means for pullingthe fiber may be the take-up spool but is preferably capstan 54. Reels51–53 are guide reels for aligning and handling the finished fiber. Thedraw force and rotational speed provided by capstan 54 determines thedraw speed of the optical fiber. Draw speeds from 1–20 meters/sec. aretypical. From capstan 54 the fiber is lead to an independently driventake-up spool 56. A stepper motor, controlled by a micro-step indexer(not shown), controls the take-up spool.

Those skilled in the art will recognize that FIG. 4 shows but one ofmany suitable arrangements.

Coating materials for optical fibers are typically urethanes, acrylates,or urethane-acrylates, with a UV photoinitiator added. The apparatus isFIG. 4 is shown with a single coating cup, but dual coating apparatuswith dual coating cups are commonly used. In dual coated fibers, typicalprimary or inner coating materials are soft, low modulus materials suchas silicone, hot melt wax, or any of a number of polymer materialshaving a relatively low modulus. The usual materials for the second orouter coating are high modulus polymers, typically urethanes oracrylics. In commercial practice both materials may be low and highmodulus acrylates. The coating thickness typically ranges from 150–300μm in diameter, with approximately 240 μm standard.

In a conventional draw apparatus the fiber essentially moves in a singleplane at least between its point of origin in the furnace and thecapstan, and no twist is intentionally impressed on the fiber. See FIG.5, which is a schematic top view of the reels in FIG. 4.

According to the invention, a torque is applied to the fiber such that aspin is impressed on the fiber. Although in principle the torque couldbe applied at any downstream point (prior to take-up) at which the fiberhas cooled sufficiently to be contacted, physical contact with barefiber should be avoided. Thus, the torque advantageously is applied at apoint downstream from the curing station, typically by means of thefirst guide roller.

We have discovered that an intermittent torque can be applied to thefiber, such that a twist with non-constant pitch is impressed on thefiber. In a preferred embodiment, this is accomplished by changing theorientation of guide roller 51 of FIG. 5, for example by canting theroller by an angle Θ around a direction parallel to the draw tower axis(see FIG. 6). Canting roller 511 as indicated causes the fiber tooscillate back and forth on the roller, in response to lateral forcesthat automatically arise in this arrangement. More specifically, thelateral forces translate into a torque on the fiber, which causes thefiber to roll laterally on roller 511, thereby moving the fiber out ofthe plane defined by the fiber in the (un-canted) apparatus of FIG. 5.It will be appreciated that the lateral roll is superimposed on theconventional draw motion. The lateral motion of the fiber is believed togive rise to a restoring force that increases with increasing lateraldisplacement of the fiber, causing the fiber to jump back(substantially, but not necessarily exactly) into the plane, only toimmediately begin another sidewise roll. This non-symmetricalback-and-forth motion is indicated by the double-headed arrow adjacentto roller 511 in FIG. 6. The angular rotation speed of the fiber duringthe lateral roll is, inter alia, a function of the cant angle Θ. Thus,the pitch of the spin impressed on the fiber is also a function of Θ.For instance, one suitable draw apparatus yielded average pitches of 14and 7 cm for Θ=7° and 15°, respectively. It will be appreciated thatthese values are by way of example only, since the pitch will depend,inter alia, on the configuration and height of the draw tower, the drawspeed, the draw tension and the coating viscosity.

Those skilled in the art will recognize that in the example justdescribed there is not only a spin impressed on the fiber but also asubstantially equal and opposite (generally elastic) twist is introducedinto the taken-up fiber. Although such fiber may be acceptable for somepurposes (e.g., for sensor purposes that require only a relatively shortlength of fiber), it will often be desirable to avoid the unwantedelastic twist. The elastic twist can be removed by appropriaterespooling. However, it is preferable to substantially preventintroduction of the elastic twist. This can be accomplished byalternately imposing a clockwise and a counterclockwise torque on thefiber as described below.

Causing the guide roller 512 of FIG. 7 to oscillate about an axis thatis parallel to the fiber draw direction (which is typically the same asthe draw tower axis) alternately impresses positive and negative spin onthe fiber. Furthermore, the resulting positive and negative elastictwists on the fiber substantially cancel, such that the fiber on thetake-up spool is substantially free of torsional elastic strain. Guideroller 512 of FIG. 7 can be caused to oscillate back and forth by anyappropriate means, e.g., by eccentric drive means (not shown). Analternate arrangement is schematically shown in FIG. 8, wherein guideroller 513 is caused to move back and forth axially, by appropriateconventional means that are not shown, resulting in alternateapplication of clockwise and counterclockwise torque on the fiber.

FIGS. 9 and 10 show experimental data comparing 850-nm fiber bandwidthfor twisted circular core fiber vs. non-twisted circular core fiber. Forthe data shown in FIG. 9, the fibers were drawn using the same apparatusand draw process. For the data shown in FIG. 10, the fibers were alsodrawn using the same apparatus and draw process but the apparatus wasdifferent from that used to obtain the data of FIG. 9. This experimentshows, inter alia, that the qualitative results obtained are notdependent on the draw apparatus.

With reference to FIG. 9, the curve with circles 61 and the curve withpentagons 63 are for fibers twisted during draw according to theinvention. The twist period for these fibers was 6.7 cm. The nominalcore ovality was less than 5%. The curve for the squares 62 givesbandwidth data for essentially the same fiber that was not twistedduring draw. Similarly, in FIG. 10, the curve with circles 65 shows datafor a fiber that was twisted during draw according to the invention witha twist period of 6.7 cm. Curves 66 and 67 show data for similar fibersthat was not twisted. The nominal core circularity in all cases was lessthan 5% ovality.

As will be understood from the above, the invention is directed toimproving the transmission properties of optical fiber that is drawnfrom preforms of high quality, i.e. free of defects such as excessivecore eccentricity or ovality. The observation that optical fiber drawnfrom these apparently relatively defect free preforms can be improvedusing twist is unexpected. The cores of these preforms typically havevery low ovality, a prerequisite for categorizing the preform as highquality. Ovality of the core can be measured by taking two or morediameter measurements, at different places around the periphery of thecore, and comparing them. Variations of less than 6%, and preferablyless than 3%, are considered as indicating a circular core. The lessevident perturbations, for example azimuthal variations, are believed tobe responsible for the discovered effect, i.e. increased bandwidth inoptical fibers drawn from preforms with high apparent uniformity andperfection.

As indicated above, the twist, or chiral structure, can be imparted tothe optical fiber by twisting the preform during draw, or twisting thedrawn fiber during draw. Each of these alternatives is meant to bedefined by the phrase “twisting the drawn fiber during the drawoperation” or the like.

Also as indicated earlier, the optical fibers that will typically beprocessed according to the invention are silica-based optical fibers.Silica-based generally means that at least 65% of the glass in theoptical fiber is SiO₂.

The twist described above is relatively constant, i.e. the same twistmotion is imparted to the entire length of fiber during the fiber draw.There are additional random mechanisms possible that may aid insubstantially reducing DMD in multimode fibers. The twisting mechanismused to impart twist to the fiber has several characteristics. Two ofnote here are the spin frequency, i.e. the number of twists per meter,the spin velocity function, which is the rate of change of spin withtime. Each of these can be randomized for additional improvement in DMDproperties. The spin frequency may be varied randomly between, e.g.,1–10 twists per meter. An arbitrary interval may be selected, forexample, 10 meters, and the twist for one ten meter segment may be 4,for another 7, and so on. A simple random number program may be used forthe control. The spin velocity function may also be randomly varied. Thespin velocity function is the axial pattern of the twisting or,equivalently the time-dependence of the angular velocity of thespinning. As shown earlier, the spin alternates in direction every meteror two, so the angular velocity of spinning crosses through zeroperiodically. A typical pattern approximates a sinusoid. However, thespin pattern may be varied to provide greater randomness. One suchpattern approximates a trapezoid with a dwell at the highest angularvelocity. Other non-sinusoidal spin patterns may be used. The spinpattern may also be varied during draw to effect additional randomness.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. An article comprising multimode optical fiber having: i anessentially circular core ii a radially varying refractive index, andiii a core diameter greater than 30 microns, with a twist impressed onthe core, the twist being at least one per meter of optical fiber. 2.Article according to claim 1, wherein the twist alternates betweenclockwise twist and counterclockwise twist.
 3. Article according toclaim 2, wherein the twist is at least four per meter of optical fiber.4. Article according to claim 2 wherein the core has an ovality of lessthan 6%.